<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="./jquery.js"></script>
    <style>
        #dv {
            width: 100px;
            height: 100px;
            border: 1px solid springgreen
        }
        
        .add {
            width: 50px;
            height: 50px;
            display: block;
            background: steelblue
        }
    </style>
</head>

<body>
    <div id="dv"></div>
    <ul>
        <li>list item 1</li>
        <li>list item 2</li>
        <li>list item 3</li>
    </ul>
    <p>a paragraph</p>
    <script>
        $('#dv').add('<p>aaaaa</p>').addClass('add')
        $("li").add("<p id='new'>new paragraph</p>")
            .css("background-color", "red");
    </script>
</body>

</html>